Design of DC Machines Interview Question Answer - 9

Important Questions on Design of DC Machines

In this post, some of important questions of Design of DC Machines are given. It includes voltage between commutator segments, selecting number of slots, number of armature slots, pulsating losses, current density in the armature conductor, factor affecting slot size, wide open slot, depth of slot, slot depth to width, resistance offered during commutation, linear commutation and delay time.

Voltage between Commutator Segments

What should be voltage limit for adjacent commutator segments at no load and full load?

  • The voltage limit for adjacent commutator segments at no load is limited up to 15 V and it is about 30 V at full load.

Which are the factors to be considered while selecting number of armature slots?

Factors to be considered for armature slots

  • Flux pulsation
  • Cooling of armature conductors
  • Mechanical difficulties
  • Commutation

Effect of Armature Slots

Describe the effect of number of slots on the cooling of armature conductor.

Number of armature slots

  • If there are large number of slots for a given armature diameter, number of conductors per slot is less. 
  • Therefore, the cooling condition is better for large number of armature slots.

Pulsating Losses

Describe the effect of number of slots on the pulsating losses.

Number of slots – Pulsating losses

  • The air gap flux due to number of slots give rise to eddy current losses in the pole faces and produces magnetic noise. 
  • As the number of slots increase, noise level of the machine and pole face losses are reduced.

Why the current density of the armature conductor should be selected high?

Current density – Armature conductor

  • The size of conductor reduces for higher value of current density for a given current. 
  • This will reduce cost of armature copper conductor. 
  • As the size of conductor reduces, the slot area requires for the conductor also reduces. 
  • Therefore, the shallow slots give better commutation condition.

Slot Size & Width

Which are the factors to be considered while deciding the slot size?

Factors to be considered for slot size

  • Reactance voltage
  • Mechanical difficulties
  • Eddy current loss
  • Flux pulsation
  • Excessive flux density

Describe the effect of slot opening on the flux pulsation in the DC machine.

  • The wide-open slot produces higher flux pulsation than the narrow open slot. 
  • This will cause iron losses and poor commutation condition therefore it is preferred that the slot open width is not so wide.

Slot Depth

Describe the importance of depth of slot.

Depth of slot

  • As the depth of slot increases, the specific permeance becomes high. This will result in high reactance voltage.

What should be limit for slot depth to slot width in the DC machine?

Slot depth to width

  • As the reactance voltage increases, the slot depth to slot width in the non inter polar machine is limited to 4. 
  • The deeper slots are allowed in the inter polar machines because inter pole produces commutating field which compensate reactance voltage.

On which factors the heights of the pole depends in DC machines?

The height of the pole depends upon mmf required for the pole and yoke at full load.

Comment : The shunt field winding and series field winding occupies same space in the compound machine.

The series field winding occupies only 20% space whereas the shunt field winding occupies 80% space in the compound machine.

Which type of resistance offered during commutation.?

Resistance offered During Commutation

  • Brush contact resistance
  • Resistance between commutator segments
  • Resistance of risers
  • Armature coil resistance

Whether the statically or dynamically induced emf during process of commutation?

Therefore, are following emfs produce during commutation.

Statically induced emf

  • This emf is produced because of the winding coil has self-inductance owing to slot leakage and overhang leakage.

Dynamically induced emf

  • The effect of armature reaction produces the field at brush axis. This emf is called as dynamically induced emf.

Linear commutation

Explain the term : Linear or straight-line commutation

Linear commutation

  • If the brush contact resistance is kept very high, the current in the coil follows straight line commutation.
  • It means that the current reversal completes in the commutation time.

Explain the term : Delay time

Delay time

  • It is time elapses between start of commutation between two adjacent armature coils.

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